Circularly polarized antennas serve a variety of satellite-communications applications, including geostationary satellites providing narrow-scan earth coverage. Although many antenna configurations are available for the task, a traveling-wave antenna developed by AK Electromagnetique (Les Coteaux, Quebec, Canada) provides the medium gain needed in an easy-to-manufacture microstrip design. An initial model has been built for L-band space applications from 1.52 to 1.67 GHz with 15 dBic gain and 17 dB return loss.

Anennas for circular polarized, medium-gain applications include horns, helix, and dielectric-rod designs, but most have significant longitudinal dimensions. The low-profile traveling-wave antenna is fairly simple in construction, consisting of a microstrip line set 3 cm above a ground plane. It is fed at one end and terminated at the other by means of a load.

The traveling-wave antenna was evaluated with a radome in an anechoic chamber (Fig. 1). The height of the antenna, including the radome, is only 4 cm. The diameter of the radome matches that of the ground plane, at 33.5 cm. The antenna uses two-turn microstrip conductor with six to eight branch lines. The 3-mm-wide microstrip is printed on 0.1-mm-thick dielectric substrate. The microstrip substrate is placed on the ground plane by a 20-mm-thick layer of Rohacell dielectric foam with permittivity (εr) of 1.04. The length of the branched lines is selected so that the current phase along the line from one branch to next is 2π + π/3 . The electric field radiated by each branch rotates by π/3 from one branch to the next.

The antenna contains 36 to 44 bends, to cancel reflections and achieve broadband input impedance match. The diameter of the ground plane including radome is less than 1.8λ and the total length of the two-turn line is about 15λ. For proper matching, the end of the microstrip line is connected to a load with a transformer placed at the input of the traveling-wave antenna. The polycarbonate radome provides environmental protection for the compact antenna.

Figure 2 shows the measured radiation pattern of the antenna at 1.57 GHz. This type of antenna has been also designed and tested for 4 GHz. The antenna provides a return loss of better than 16 dB for a 15-percent bandwidth at 4 GHz.